A series of sulfhydryl compounds have been developed by Walter Reed Army Institute, which protect normal tissues against radiation injury. WR 2721, one drug in this series, gives a dose modifying factor (i.e., ratio of radiation doses that give the same level of biological damage with the drug as without the drug) ranging from 1.2 to 2.5 depending on the normal tissue tested. Although this drug protects some murine tumors against radiation, the DMF in tumors is smaller than in tissues. This drug is now being used in clinical trials in radiation therapy. The purpose of this work is to: 1. study the ability of this drug to protect a rapidly proliferating and slowly proliferating tissue, i.e., skin and lung, respectively, in mice; 2. to determine optimum timing of drug administration; 3. to determine if WR 2721 protects at small doses/fx in the clinical range; 4. to determine if there is a higher DMF for late than for early damage in mouse lung.